Currently, a transmission rate in a high-speed optical fiber transmission system has been increased, for example, from 40 Gbit/s to 100 Gbit/s, and even to 400 Gbit/s. However, various effects in the optical fiber transmission system such as a nonlinear effect, a polarization mode dispersion (PMD) effect, and a differential coding cost have severely limited a transmission distance of the high-speed optical fiber transmission system. It is well known that, action processes of these damage effects may be described by using trellis diagrams, and therefore, these damage effects may be compensated for to some extent by using a BCJR (Bahl, Cocke, Jelinek, and Raviv) compensation algorithm, a forward and backward recursive operation algorithm.
To further compensate for the limit on the transmission distance of the high-speed optical fiber transmission system from the effects in the optical fiber transmission system, it has been proposed that compensation is performed in a Turbo equalization manner at a receive end of the high-speed optical fiber transmission system, in other words, system performance is improved by means of interactive iteration between multiple soft information processing modules. For example, iteration is performed between a low density parity check (LDPC) decoder and a BCJR module, to compensate for the differential coding effect, the nonlinear effect, the PMD effect, and the like. Such a Turbo equalization manner can greatly improve the system performance by compensating for damage in a channel. Herein, one-bit soft information refers to a probability value of decision on whether this bit is 0 or 1. To simplify an operation, generally a ratio of a probability of decision that the bit is 0 to a probability of decision that the bit is 1 is used, and then the logarithm of the ratio is taken.
In addition, a Turbo equalizer implementing the foregoing Turbo equalization manner uses a feedback structure and an LDPC code word, and a BCJR module of the Turbo equalizer uses a common sliding window BCJR with a serial structure. Generally, a length of the LDPC code word used in optical communication reaches ten thousand bits, and the entire LDPC code word needs to be stored in the BCJR module. Therefore, the BCJR module may have numerous storage resources. However, the Turbo equalizer using the feedback structure, the LDPC code word of a large length, and the complex BCJR module all limit a system throughput.
It can be seen that, in an optical fiber transmission system with a high speed greater than 100 Gbit/s, to implement a high throughput greater than 100 Gbit/s, the foregoing Turbo equalization manner cannot adapt to large-capacity high-speed transmission.